When Bassanio is about to make his choice of the three caskets at Belmont - a choice upon which rests his chances of marriage to Portia - she calls for music and singing while he ponders.
Music, whilst BASSANIO comments on the caskets to himself
Tell me where is fancy bred,
Or in the heart, or in the head?
How begot, how nourished?
It is engender'd in the eyes,
With gazing fed; and fancy dies
In the cradle where it lies.
Let us all ring fancy's knell
I'll begin it,--Ding, dong, bell.
-- The Merchant of Venice, Act 3, Scene 2, Lines 64-73
Is this song an attempt by Portia to influence Bassanio subconsciously towards lead?
The meaning of the song is about "fancy", superficial liking; it suggests that what appeals to the eye is not necessarily what is truly valuable. And in a subtle piece of wordplay, the first three lines of the song all rhyme with "lead". Portia is already in love with Bassanio, although she dare not admit it, and she's certainly clever enough to set up such a scheme, in order to prod him towards choosing the right casket but still maintain plausible deniability. On the other hand, perhaps I'm reading too much into this, and underestimating Portia's honour and dedication to the rules imposed by her late father.